Alloy steel



good weldability,

- quenched and tempered constructional alloy steels.

3,132,025 ALLOY STEEL John L. Hurley, Bloomfield, N..l., assignor to TheInternational Nickel Company, Inc, New York, N.Y., a corporation ofDelaware Filed Dec. 3, 1962, Ser. No. 241,535 7 Claims. (Cl. 75-124) Thepresent invention relates to low alloy steels and, more particularly, tolow alloy constructional steels in the hot-rolled condition whichmanifest an excellent combination of metallurgical properties includingstrength, formability, toughness, weldability, and corrosion resistancewhich render the steels eminently suitable for divers applications.

A hot-rolled steel from a commercial viewpoint should be capable of usein a tremendous number of diiierent applications as opposed to findingpossible recourse in the utilization of many difi'erent and varyingsteel compositions (or those requiring heat treatments) for the sameapplications. This requires a steel of low cost which can be easilyprocessed and which is characterized by a combination of properties,each of which does not fall below a minimum requirement. A particularsteel may possess certain properties which are quite good, but at thesame time be afiiicted with one or more inferior properties which wouldlimit its application. If a hot-rolled steel is to be versatile ofapplication, as in accordance with the present invention, it shouldpossess (1) a yield strength of at least 70,000 p.s.i., (2) good impactproperties at relatively low temperature as well as at room temperatureand above, e.g., over 15 foot-pounds (ft-lbs.) at 50 F. and over atleast 75 ft.-lbs. at room temperature as measured by the Charpy V-notchtest, (3) a tensile elongation of over 20%, (4) good formability,including a reduction of area of at least about 60%, (5) and (6) goodcorrosion resistance characteristics. In addition, since structuralapplications are a primary use of steels in the as-rolled condition,such steels should possess a ratio of yield to tensile strength of atleast 0.8 and preferably at least 0.85. Such ratios assure a designerthat lesser amounts of material can be safely employed than otherwisemight be the case.

If the prior art structural steels (other than the low strength plaincarbon steels) could be. categorized, at least two major classes wouldevolve, to wit, (1) the hotrolled, high strength low alloy steels, and(2) the water The former, as marketed commercially, generally have ayield strength on the order of about 50,000 to 55,000 pounds per squareinch (p.s.i.) in the hot-rolled condition, a strength considerablysuperior to that of the plain hotrolled carbon steels, but quiteinsufiicient for far too many commercial applications. Higher strengths,e.g., 65,000 p.s.i., have been attained through hardening.(precipitation) treatments but it has been found necessary to exercisevarious precautions to avoid theoccurrence of undesirable results. Forexample, embrittlement is an ever present obstacle attendant theprecipitation treatment and to minimize this problem, molybdenum with orwithout aluminum has been considered necessary. Further, it has beencommon practice to normalize prior hot-rolled steels to efifect improvedproperties, e.g., impact strength,

. but this treatment is tedious and expensive and it would as is wellknown, high silicon contents result in the so United States Patent 0"ice termed dirty steels which, from the commercial viewpoint, aredifiicuit and undesirable to process. Moreover, high silicon contentscan adversely affect impact strength and the tensile elongation of suchsteels is marginal (15 %20%) in the hot-rolled condition. To improvesuch properties, normalizing and stress relieving operations have beenproposed.

Regarding the quenched and tempered structural steels, yield strengthsof 80,000 psi. upwards to 110,000 p.s.i. have been attained but theprocessing of such steels is expensive and attended by much difiiculty.For example, the cost of such steels is increased by the necessity ofinstalling and operating equipment capable of affording strictoperational control in the austenitizing, quenching, and temperingoperations. Further, liquid quenching, being a severe and drastic typeof quench, is causative of or promotes buckling and distortion, thusundesirably limiting the shapes that can be produced or necessitatingthe utilization of elaborate jigs for quenching. Still further,ductility (tensile elongation), for example, is about 20% for suchalloys and while this may be adequate for various forming operations, itis at best marginal for many others. Formability is of tremendousimportance to exploit the strength of the steels, e.g., the bucklingstrength of a formed structural member in large measure is dependentupon the smallest radius of bending that can be used. In addition, inwelding such steels, special lowhydrogen fluxes are necessary to avoidunderbead cracking. Should the most economical submerged arc weldingmethods be used, lower yield strengths result and embrittlement of theweld heat-affected zone is encountered.

It has now been discovered that special low alloy steels of particularcomposition can be provided in the hotrolled condition and which arecharacterized by a highly satisfactory combination of propertiesincluding each of those specified hereinbefore. Thus, the steels areespecially suitable as structural steels for general application.

It is an object of the present invention to provide a new and improvedlow alloy constructional steel in the hot-rolled condition andcharacterized by the combination of properties referred to hereinbefore.

Other objects and advantages Will become apparent from the followingdescription.

Generally speaking and in accordance with the present invention, thecombination of properties referred to herein is readily obtained withhot-rolled, low alloy steels of the following most advantageouscomposition based on weight percent: carbon in as small amounts asfeasible up to 0.08%, e.g., about 0.01% to 0.06% carbon, about 0.2% toabout 0.75% manganese, silicon in an amount suflicient for deoxidationpurposes, e.g., 0.05%, and up to 0.35%, about 1% to about 1.7% copper,about 0.7% to about 1.6% nickel, about 0.01% to about 0.16% columbium,and the balance essentially iron. Aluminum in amounts to achieve gooddeoxidation, e.g., up to 0.1%, as is consistent with commercialsteelmaking practice, is

advantageous. Up to 0.5% aluminum can be employed but because suchamounts can lead to high inclusion content, it is preferred that thealuminum content not exceed 0.1%. Chromium is not required, and, in anyevent, it

' should not be present in an amount higher than 0.2% since higheramounts, e.g., 0.3%, impair impact strength. Sim ilarly, molybdenumshould be kept below 0.2% since higher amounts can adversely aiiect boththe ratio of yield to tensile strength and impact properties in theas-rolled condition. Sulfur and phosphorus should be kept as low as iscommercially practical.

In the hot-rolled condition, steels within the foregoing ranges aifordyield strengths well in excess of 70,000 psi. together with yield totensile strength ratios of at least 0.8 and up to well over 0.9, hightensile elongations (1.4" gage length) of at least as high as and up toat least exceptional formability as evidenced by reductions of area ofover and excellent impact toughness as measured by the standard CharpyV-notch tests, i.e., impact strengths of at least 15 ft.-lbs. at 50 F.and ft.-lbs. to as high as over 200 ft.-lbs. at room temperature.Further, the steels are very readily weldable and manifest goodcorrosion resistance. In addition, the alloy steels are easilyprocessable and are of relatively low cost. For example, normalizingand/or stress relieving treatments are not at all required as has beenthe case with many prior art steels. These additional considerationsrender the alloy steels particularly attractive from a com mercialviewpoint.

A further advantage of the alloy steels within the instant invention isthat they can be hardened (as will be shown hereinafter) by a verysimple precipitation hardening treatment. This treatment results in ahigher yield strength level, a strength level which would be ofparticular advantage for certain applications.

For continuously achieving optimum results, it is important that theforegoing compositional ranges be observed. However, satisfactoryresults can be achieved with alloy steels having the following ranges:carbon in an amount up to 0.08%, about 0.1% to about 1% manganese,silicon from about 0.03% to less than 0.5%, about 0.9% to about 2%copper, about 0.5% to about 2.5% nickel, about 0.005% to about 0.25%columbium and the balance essentially iron.

in accordance with the invention, it has been found that the yieldstrength of the alloy steels is virtually insensitive to carbon content.That is to say, the alloy steels of the present invention are notdependent upon carbon content for strength as is the case with manyother prior art steels. A small amount of carbon, e.g., 0.01%, isnecessary to combine with the Columbium upon cooling from the hotworkingoperation in order to achieve a small ferrite grain size. While it ismost advantageous that the carbon content not exceed 0.06%, there arecommercial applications which do not require optimum formability, impacttoughness, and weldability (which are afforded by alloy steels withinthe invention in which the carbon content advantageously does not exceedabout 0.06%). In such cases, the carbon content can be extended up toabout 0.19%. Because of the extremely low carbon content that can beemployed and because of the virtual insensitivity of yield strength tocarbon content, the steels can be decarburized during processing withoutincurring a detrimental loss of strength.

With respect to the manganese content of the alloy steels, at least 0.1%should be present to obviate fabricating diificulties attributable tosuch elements as sulfur and to avoid cracking during hot rolling. It ispreferred that the manganese be less than about 1% since it has beenfound that higher amounts adversely affect impact properties. Siliconshould be kept as low as is consistent with commercial steelmakingpractice. As mentioned hereinbefore, high amounts of silicon, e.g.,0.75%, lead to a steel which contains an undesirably high number ofinclusions which can adversely affect certain desired properties. Copperis a most essential element in accordance with the invention and whereprecipitation hardening is desired, at least 1% copper should bepresent. However, the presence of copper in appreciable amounts above1.7% tends to cause cracking during hot rolling. Nickel has been foundto be very beneficial in preventing cracking during hot rolling, but ifthe nickel content appreciably exceeds about 2.5%, i.e., over 3%, thealloy steels of the invention would have a tendency to formtransformation products other than ferrite (and possibly small amountsof pearlite) during cooling from the austenitic state. This can lead toundesirable difiiculties. Further, to avoid cracking during hot working,the ratio of copper to nickel should not appreciably exceed a ratio of 2to 1.

To achieve optimum strength, columbium is important since hot-rolledsteels devoid of columbium are quite inferior in various properties,particularly yield strength. Columbium is not present in the hot-rolledsteels of the invention to repress the growth of austenite grain size bycarbide action as would be the case, for example, in carburizing steels.Rather, columbium must first, in order to obtain the enhanced propertiesin accordance with the invention, be dissolved in the steel duringheating prior to hot rolling. It will be appreciated that heating of thesteels to high temperatures, e.g., over about 2000 F, is necessary inaccordance with the present invention in order for the hot rolling and/or hot working operations to be carried out successfully. The austenitegrain size increases when the steels are heated to rolling temperaturebut it is broken down during hot working. It is considered that thebeneficial effects of columbium are actually imparted upon thetransformation of the steel from austenite to ferrite during cooling ofthe hot-rolled steels from austenitizing temperatures. Thetransformation reaction occurs so rapidly upon air-cooling that it actssomewhat as a triggering mechanism for columbium in its role ofproviding a small ferrite grain size, e.g., A.S.T.M.

I No. 9 or 10 or smaller, and of imparting increased strength andtoughness to the steels. The following conditions are necessary toachieve this rapidly occurring transformation: the steels must be in theaustenitic condition, the Columbium must be dissolved in the austenite,the austenite must be subjected to heavy plastic deformation, e.g., byhot rolling, hot working, forging, etc., and the steels, after the hotrolling operation, must be cooled through the transformation range at arate approximating that employed in the air-cooling of a steel plate.The more rapid the rate of air-cooling, the lower is the tempera ture atwhich ferrite forms and this, in turn, provides a finer grain size.Further, with low carbon contents the transformation temperature of thealloy steels is compar-atively high. Since it is common to employ afinishing pass at, say, about 1650 F. in the production of hotrolledsteels, initiation of the transformation to ferrite is shortened withsteels having high transformation temperatures. While it is muchpreferred to employ columbium, vanadium can be used to replace Columbiumin whole or in part.

For the purpose of giving those skilled in the art a betterunderstanding of the invention and/or a better appreciation of theadvantages of the invention, the following illustrative description anddata are given:

Several alloy steels having compositions within the ranges set forthabove were prepared and are identified in Table I.

Table 1 Alloy No 0, Mn, Si, Ni, On, A], Ob,

percent percent percent percent percent percent percent;

The above steels were melted (about 30 1b. melts) in an a air inductionfurnace and subsequently forged to plate 3 precipitation hardenedcondition, the latter involving a a heating for one hour at 1-000 F. andair-cooling (after hot rolling and cooling). The tensile propertiesgiven in Table II represent the average of duplicate 0.357-inch diametertensile bars which were cut from the /2-inch HOT ROLLED-AIR COOLEDseverely during hot rolling. As indicated previously herein, the nickelcontent should be present in amounts of at least about 0.5% and, mostadvantageously, at least 0.7%. In both the copper-free andcolumbium-free com- Yield '1. S.,1 Red. Oharpy V-notch Impact, Ft.-lb.Alloy N0. Point, 191., in

10, p.s.i. Percent Area, p.si Percent 70 F. 0 F. 50 F.

77.5 30.9 31. 0 75.2 177, 230 145, 221 174, 134 77. 1 7s. 2 55.2 77. 0155, 201 155, 199 124, 178 75.4 80.7 27. 5 70. 5 149,140 155,101 112, 5979. s 91. 2 25. 0 53. s 135, 39 54, 74 21,44 75.8 80. 9 33. 2 71. 5 13s,13s 95, 100 73, 44 75.5 27.9 30.5 54. 5 73, 2 91,50 53, 50 74.9 91. 723. 5 59. 0 s2, 54 29, 55 22, 2s 73. 5 s2. 5 30. 5 67.8 173,137 129,133107, 95 as. 9 s9. 5 26.8 59. 3 122 84, 70 45, 55 73.4 84.0 29. 5 71.8123 55 87 42,101

HOT ROLLED 1,000 F., 1 HOUR 95.9 30.1 74. 5 130,85 g s3, 32 94. 3 a5. 074.2 157,150 124, 58 75, 52 95. 5 24. 9 57.3 151,102 32, 54 7, 7 98.625. 7 51.5 54, 45 22, 35 14, 7 98.5 23. 5 57.3 110,45 28 17,14 101.2 25.2 51. 5 70, 57 21, 15, 7 104. 3 25. 5 57. 4 49, 50 23, 21 9, s

1 Tensile strength. 2 Elongation in 1.4 gage length.

Table II illustrates that each of the alloy steels manifested a yieldpoint of well over 70,000 p.s.i. in the hotrolled condition, a strengththat could be further im .proved by the application of the simpleprecipitation .Thus, for optimum results it is advantageous that thecarbon content hot exceed about 0.06%.

The data in Table II' are further illustrative of the excellentelongation, reduction of area, and impact strength properties of thealloy steels in the hot-rolled condition. Apart from alloy No. 7, eachof the alloy steels mani tested a ratio of yield to tensile strength ofgreater than 0.85, an elongation of at least 25%, a reduction of area ofat least 60%, andla Charpy impact resistance of at least 15 ft-lbs.(over 20 ft.-lbs.) at 50 and at least 75 -t.-lbs. at room temperature.Actually, the reduction of area data is a better indicator of tensileductility rather than elongation, because at least one of the pairs ofspecimens tested (except alloys No. 4, 5 and 6) deformed extensivelyoutside the gage limits and, thus, the actual elongation data given inTable II for these alloys, while extremely good, are below the truevalues. As will be appreciated, hot-rolled alloy steels within theinvention are exceedingly tough as illustrated by alloys No. 1 and 2which are characterized by elongations of over reductions of area ofover 70%, and impact resistances of well over 100 ft.-lbs. at 50 F.

In addition to the foregoing data, alloys outside the scope of theinvention were prepared and tested under the same conditions outlinedhereinabove. Three of such alloys had compositions within the mostadvantageous range except that either nickel or copper or columibum wasnot added to the steels. The nickel-free steel cracked parable steels,it Was found that the yield strength was less than the required minimumof 70,000 p.s.i. In addition, a chromium-containing steel otherwisewithin the composition limits of the invention except that it contained0.33% chromium, was prepared and tested. At 50 F. this steel had CharpyV-notch impact values of 7 and 14 lbs. This illustrates the importance(as referred to hereinbefore) of maintaining the chromium content (ifadded or introduced by Way of scrap) low.

That the alloy steels are readily weldable is demonstrated by thefollowing submerged arc weld test: a plate specimen to be Welded wasprepared with the top edges .being hand ground to a bevel inch wide witha 30 to 40 degree angle. A nickel-copper alloy steel wire inch indiameter was used as the welding wire together with a suitable flux.Using a current of about 600 amperes, a voltage of about 30 volts and adeposition rate of 12 inches per minute, one pass on each side of theplate was made. Inspection showed the welds to be crack free.

For applications requiring a hardened alloy steel, the alloy steels ofthe present invention can be hardened by subjecting them to atemperature of between about 850 F. to about 1150" F., e.g., 950 F. toabout 1050 F., for a period sufiicient to effect the required degree ofprecipitation hardening. For example, one hour at 1000 F. is quitesatisfactory. A shorter period of time can be employed at the higherlevels of temperature (1050- F. to 1150 F.). Conversely, if the alloysteels are heat treated at, say, 850 F. to 950 F., a period greater thanone hour might be necessary. A particular benefit of the precipitationhardened steel is that a tenacious scale is formed during theprecipitation treatment and this adherent scale can form an excellentbase for a protective coating, e.g., paint.

In view of the combination of metallurgical properties characteristic ofthe alloy steels within the present invention, the steels can be used ina wide variety of applications such as automobile bumpers, truck frames,tubular vessels, railroad cars, structural components including I-beams,rolled form shapes, etc. The alloy steels are also suitable for use inapplications for moderate temperature use, i.e., above room temperatureand up to about 400 F. or 500 F.

As will be readily understood by those skilled in the art, the termbalance when used to indicate the amount of iron in the alloy steelsdoes not exclude the presence of other elements commonly present asincidental elements, e.g., deoxidizing and cleaning elements, andimpurities ordinarily associated therewith in small amounts which do notadversely afiect the basic characteristics of the steels. In addition,the terms hot-rolled or asrolled as used herein are intended to include,as those skilled in the art will readily understand, such operations asthe application of finishing passes or temper rolling.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to Without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

I claim:

1. An alloy steel characterized in the hot-rolled condition by a yieldstrength of at least 70,000 p.s.i., a yield to tensile strength ratio ofat least 0.85, a Charpy V-notch impact strength of at least 15 ft.-lbs.at 50 F. and at least 75 ft.-lbs. at room temperature, a tensileelongation of over 20%, a reduction of area of at least 60% and goodweldability and corrosion resistance, said alloy steel consistingessentially of carbon in an amount up to 0.06% about 0.2% to about 0.75%manganese, silicon in an amount up to 0.35%, about 1% to about 1.7%copper, about 0.7% to about 1.6% nickel, the ratio of copper to nickelnot exceeding about 2 to 1, about 0.01% to about 0.16% columbium,aluminum in a small but effective amount up to about 0.1% sufiicient toinsure good deoxidation, and the balance essentially iron.

2. An alloy steel characterized in the hot-rolled condition by a yieldstrength of at least 70,000 p.s.i., a yield to tensile strength ratio ofat least 0.85, a Charpy V-notch impact strength of at least 15 ft.-lbs.at 50 F. and at least 75 ft.-lbs. at room temperature, a tensileelongation of over 20%, a reduction of area of at least 60%, goodweldability and corrosion resistance, said alloy steel consistingessentially of carbon in an amount up to 0.08%, about 0.2% to about0.75% manganese, silicon in an amount up to about 0.35%, about 1% toabout 1.7% copper, about 0.7% to about 1.6% nickel, the ratio of copperto nickel not exceeding about 2 to 1, about 0.01% to about 0.16%columbium, aluminum in a small but eifective amount up to 0.1%sufiicient to insure good deoxidation, and the balance essentially iron.

3. An alloy steel in the hot-rolled condition characterized by a yieldstrength E at least 70,000 p.s.i. together with a yield to tensilestrength ratio of at least 0.85, a Charpy V-notch impact strength of atleast ft.lbs. at

F. and at least 75 ft.-lbs. at room temperature, a tensile elongation ofover 20%, a reduction of area of at least good weldability and corrosionresistance, said alloy steel consisting essentially of carbon in anamount up to 0.08%, about 0.1% to about 1% manganese, silicon from about0.03% to less than 0.5 about 0.9% to about 2% copper, about 0.5% toabout 2.5% nickel, the ratio of copper to nickel not exceeding about 2to 1, about 0.00-5% to about 0.25% columbium, aluminum in a small buteffective amount up to 0.5% sufficient to insure good deoxidation, andthe balance essentially Iron.

4-. An alloy steel consisting essentially of carbon in an amount up to0.19%, about 0.1% to about 1% manganese, silicon from about 0.03% toless than 0.5%, about 0.9% to about 2% copper, about 0.5% to about 2.5%nickel, the ratio of copper to nickel not exceeding about 2 to 1, about0.005% to about 0.25% columbium, alu minum in a small but effectiveamount up to 0.5 suflicient to insure good deoxidation, and the balanceessentially iron.

5. A hardened alloy steel consisting essentially of carbon in an amountup to 0.06%, about 0.2% to about 0.75% manganese, silicon in an amountup to 0.35%, about 1% to about 1.7% copper, about 0.7% to about 1.6%nickel, the ratio of copper to nickel not exceeding about 2 to 1, about0.01% to about 0.16% columbium, aluminum in a small but effective amountup to about 0.1% sufiicient to insure good deoxidation, and the balanceessentially iron.

6. A hardened alloy steel consisting essentially of carbon in an amountup to 0.08%, about 0.1% to about 1% manganese, silicon from about 0.03%to less than 0.5%, about 0.9% to about 2% copper, about 0.5% to about2.5 nickel, the ratio of copper to nickel not exceeding about 2 to 1,about 0.005% to about 0.25 columbium, up to 0.5 aluminum, and thebalance essentially iron.

7. A hardened alloy steel consisting essentially of carbon in an amountup to 0.19%, about 0.1% to about 1% manganese, silicon from about 0.03%to less than 0.5%,

about 0.9% to about 2% copper, about 0.5 to about 2.5 nickel, the .ratioof copper to nickel not exceeding about 2 to 1, about 0.005 to about0.25 columbium, up to 0.5 aluminum, and the balance essentially iron.

References Cited in the file of this patent UNITED STATES PATENTS2,046,168 Kinzel et al June 30, 1936 2,158,651 Becket et'al May 16, 19392,182,135 Reinhardt Dec. 5, 1939 2,443,932 Rofi et al June 22, 19483,010,822 Altenburger Nov. 28, 1961 OTHER REFERENCES Alloys of Iron andCopper, pages 97 and 160, by Gregg and Daniloif, published in 1934 bythe McGraw-Hill Book Company, New York.

1. AN ALLOY STEEL CHARACTERIZED IN THE HOT-ROLLED CONDITION BY A YIELDSTRENGTH OF AT LEAST 70,000 P.S.I., A YIELD TO TENSILE STRENGTH RATIO OFAT LEAST 0.85, A CHARPY V-NOTCH IMPACT STRENGTH OF AT LEAST 15 FT.-LBS.AT -50*F. AND AT LEAST 75 FT.-LBS. AT ROOM TEMPERATURE, A TENSILEELONGATION OF OVER 20%, A REDUCTION OF AREA OF AT LEAST 60% AND GOODWELDABILITY AND CORROSION RESISTANCE, SAID ALLOY STEEL CONSISTINGESSENTIALLY OF CARBON IN AN AMOUNT UP TO 0.06%, ABOUT 0.2% TO ABOUT0.75% MANGANESE, SILICON IN AN AMOUNT UP TO 0.35%, ABOUT 1% TO ABOUT1.7% COPPER, ABOUT 0.7% TO ABOUT 1.6% NICKEL, THE RATIO OF COPPER TONICKEL NOT EXCEEDING ABOUT 2 TO 1, ABOUT 0.01% TO ABOUT 0.16% COLUMBIUM,ALUMINUM IN A SMALL BUT EFFECTIVE AMOUNT UP TO ABOUT 0.1% SUFFICIENT TOINSURE GOOD DEOXIDATION, AND THE BALANCE ESSENTIALLY IRON.